(1) Field of the Invention
The present invention relates to an enclosed cell such as an alkaline storage battery or a lithium battery having a safety valve mechanism and methods of fabricating the same. The safety valve mechanism is operable to release gas from inside the cell when an internal pressure of the cell rises to an excessive degree.
(2) Description of the Prior Art
With an enclosed cell such as an alkaline battery or a lithium battery, it is generally an important matter from the safety point of view that its opening is sealed tightly. An insufficient sealing results in liquid leakage. In the case of a lithium cell in particular, the insufficient sealing leads to a further problem of deterioration in cell performance. This arises from the reaction of the lithium anode with the moisture in ambient air entering the cell. The reaction produces a passivation film on the lithium cathode surface which increases the internal resistance of the cell.
Various cells have heretofore been proposed with a view to improvement in sealing reliability.
FIG. 1 of the accompanying drawings illustrates one such example of enclosed cell, wherein the improvement resides in a hermetic seal structure comprising a glass or ceramic insulator a. More particularly, this enclosed cell comprises an outer canister b acting as one of the polar terminals, a metal lid c fused to the outer canister b such as by laser welding, and a cylindrical pin d inserted into and fixed to the insulator a to act as the other polar terminal. The outer canister b includses a thin wall portion e which breaks when the internal resistance of the cell rises as a result of misuse or in an abnormal environment. The internal pressure is released to the ambient surroundings through the breakage, thereby to prevent the cell from bursting.
However, this hermetic seal structure generally is costly because the insulator a is formed of glass or ceramics. It is also difficult to control the thickness t of the thin wall portion e. This impedes valve operating pressure setting, which is contrary to full assurance of safety.
FIG. 2 illustrates a second example of an enclosed cell so far proposed. To improve the sealing this cell discards the hermetic seal structure which is costly and involves difficulties from the point of view of manufacturing technique, in favor of using an insulating packing f formed of a resin (see the Japanese utility model application laid open under No. 60-22753). In this enclosed cell, a second terminal d having a T-shape section is inserted into the packing f and calked by means of a metal washer g fitted on a lower position of the terminal d. This example includes a metal lid c secured to an outer canister b by laser welding or the like as in the first known example. The resin packing f melts and forms an opening (not shown) when the internal pressure of the cell increases or the cell temperature rises to an excessive degree. The gas in the cell is released to the ambient surroundings through this opening, thereby to check the internal pressure increasing and prevent the cell from bursting.
In this enclosed cell, however, the washer g has an outside diameter g greater than the diameter i of a bore defined in the metal lid c, and the bore may be closed by the washer g when the terminal d moves in a direction of arrow j with a rise in the internal pressure of the cell. Thus, the washer g may obstruct the release of the gas from inside the cell. This cell fails to assure safety when used in a severe environment, for example at high temperature.